Acute and chronic liver infections caused by Hepatitis B virus (HBV) constitute a major worldwide public health problem. There are over 350 million chronic carriers of the virus worldwide, including 1.7 million chronic carriers in the US, who are affected by chronic hepatitis B (CHB). In addition to human suffering, the economic costs are large - more than $1 billion/year is spent for HBV-related hospitalizations in the US. Although HBV infection can be prevented by vaccination, emergence of escape mutants has been noted, there is concern that vaccines will become ineffective. Thus, there is a clear need for effective antiviral therapy. The future treatment of CHB is expected to be combination therapy with two or more direct-acting antiviral drugs with different mechanisms of action. We have discovered SB 40, as a first-in-class, small molecule nucleic acid hybrid (SMNH) with novel mechanism(s) of action. Extensive studies conducted over the past several years, (supported in part by a UO1 Grant from NIAID), have led to an oral prodrug designated as SB 44. SB 44 has direct antiviral and potential immunomodulatory properties. SB 44, (i) has multiple mechanisms of action including activation of RIG-I, a host target, hence less potential to elicit antiviral resistance, (ii) is not a chain terminator of HBV DNA synthesis; hence less potential for mitochondrial toxicity, (ii) is synergistic with other anti-HBV and anti-HCV drugs, (iv) is active against resistant strains of HBV, and (v) is a potential replacement for Interferon. Preclinical proof of concept has been demonstrated. SB 44, (i) inhibits HBV replication in cell culture studies with good selectivity index, (ii) is active against HBV and Hepatitis C virus (HCV), (iii) shows efficacy against HBV in the transgenic mouse model of HBV, (iv) suppresses HBV DNA synthesis in cells and in vivo, and unlike nucleoside and nucleotide analogs, is not a chain terminator of DNA synthesis, and (iv) stimulates expression of EEEH protein in HBV transgenic mice; hence SB 44 has potential for broad-spectrum antimicrobial activity. SB 44 has good pharmaceutical properties. SB 44 is: (i) orally available with significant disposition in the liver, the target organ for HBV and HCV, (ii) non-toxic in initial preclinical studies and has less potential for toxicity upon longer term use, and (iii) a small molecule that is readily manufactured. Given its excellent preclinical profile, SB 44 merits further development as a novel anti-HBV agent. This 5-year project will be carried out in partnership with a team of outstanding collaborators in academia and industry. Studies conducted thus far have resulted in substantial know-how, hence the project goals and defined milestones are achievable. The studies proposed in the project will help advance SB 44 to IND and human clinical trials.